1. Field of the Invention
The present invention relates to a control system for an automatic transmission for automobiles and, more particularly, to an automatic transmission control system for automobiles having a hydraulic circuit which has a low/reverse frictional coupling element locked in a reverse range, by a manual valve, and a low range of an automatic transmission.
2. Description of Related Art
Typically, automatic transmissions for automobiles have various torque converters and transmission gear mechanisms. Each such torque converter and gear mechanism includes a plurality of frictional coupling elements which are selectively locked or coupled and unlocked or released. Such coupling elements may include clutches and brakes and are locked and unlocked so as to place the automatic transmission into desired transmission modes or ranges and gears according to driving conditions. Selectively locking and unlocking of these frictional coupling elements is performed by the use of actuators associated with the respective frictional coupling elements which are controlled by means of a hydraulic control circuit.
Hydraulic control circuits of this kind typically include various hydraulic control valves, such as a line pressure regulator valve, a valve for generating hydraulic pressure corresponding to throttle openings of an engine throttle valve, a manual valve which is operated manually to change from one transmission range to another, a plurality of shift valves for selectively activating the actuators so as to provide any desired gear, and a plurality of solenoid valves for driving the shift valves selectively according to driving conditions. Such a hydraulic control circuit is known from, for instance, Japanese Unexamined Patent Publication No. 63-186,055.
Some automatic transmissions of this type include two frictional coupling elements which correlate with each other such that they cause mechanical locking of the transmission gear mechanism if they are simultaneously locked. For example, if simultaneous locking between a 3/4 clutch, disposed between a turbine shaft of the torque converter and a carrier of the planetary gear forming the transmission gear mechanism, and a low/reverse brake for locking the carrier is caused, the turbine shaft is disabled from turning, causing mechanical locking of the transmission gear mechanism. In order for a conventional automatic transmission to avoid the simultaneous locking of a frictional coupling element and the low/reverse brake corresponding to mechanical locking of the automatic transmission, the low/reverse brake is configured so as to be locked as a result of manual operation of the manual valve.
Because it is necessary for a hydraulic control circuit, which locks the low/reverse valve as a result of manual operation of the manual valve, to be organized so as to deliver hydraulic pressure into various pressure lines through the manual valve, the hydraulic control circuit is typically very complicated in structure. In addition, because the low/reverse brake is locked due to the operation of the manual valve, it is difficult to provide timely locking of the low/reverse brake. This is not good enough for the prevention of shift shock.